1. Field of the Invention
The present invention relates to the field of engineering use of superconductors. It relates to an arrangement and a method for current limiting using a superconducting transformer.
2. Description of Related Art
In a superconducting current limiter which is designed, for example, for an electrical current distribution network, use is made of the fact that a superconductor which is cooled to a temperature below the critical temperature Tc maintains its superconducting capability only for as long as the current density of a current flowing through it remains below a certain limit value. This limit value is normally referred to as the critical current density jc or, related to a specific conductor cross section, as the critical current level Ic, and is in principle dependent on the temperature of the superconductor, and on the magnetic field passing through it. If the current level in the conductor exceeds the critical value Ic in the event of a short circuit, its electrical resistance increases, and the short-circuit current is limited. In DE 44 18 050 A1, this is done using an induction coil, whose impedance rises suddenly since its iron core is no longer shielded.
One major requirement for current limiters of any type in electrical power distribution networks is that they limit a short-circuit current to a safe level, and immediately carry the related current again once the short circuit is no longer present. In particular, it should not be necessary to interrupt the circuit, so that a load which is protected by the same current limiter as the short-circuit path is not entirely disconnected from the power source.
If a superconductor is loaded with an excessive current for too long, it is heated to well above the critical temperature Tc and, in some circumstances, may be damaged. A current limiter heated in such a way also still has a considerable resistance when it now has to carry only the rated current again once the short circuit has been rectified. The subsequent cooling-down process to a temperature below the critical temperature Tc lasts for a certain minimum time owing to the physically predetermined limits relating to the heat transmission between the superconductor and the cooling medium. This is often longer than the duration of the actual short circuit, so that the power supply to the load is interrupted, or at least limited, for much longer than is necessary.
In a current limiter based on superconductors, it is thus necessary to protect against thermally overloading that superconducting section which carries the rated current without any losses during normal operation. A parallel-connecting auxiliary path is suitable for this purpose, which contributes little or nothing to carrying the current during normal operation, but helps to carry the short-circuit current.
If a superconducting transformer is provided in an otherwise normally conductive circuit, this makes it possible to equip this transformer with a current-limiting characteristic as well. U.S. Pat. No. 4,336,561 discloses such a transformer, in which parallel-connected main and auxiliary windings are provided on both the primary and secondary. The main windings are designed such that they carry the rated current, without any resistive losses, during continuous operation. The two auxiliary windings, which are designed as load-reducing elements, are not perfectly coupled, that is to say they are distinguished by a substantial stray impedance, similar to that of a free-standing induction coil. For this reason, virtually no current flows through the auxiliary windings during continuous operation. If the rated current is now exceeded in the event of a short circuit, both the primary and auxiliary main windings change to a resistive state, and the current flow is at least partially commutated onto the auxiliary windings. Their stray impedance now prevents an unlimited current rise. A switch can interrupt the main windings in the event of a short circuit, in order to protect them against overloading. The auxiliary windings are designed such that they remain superconducting in all circumstances, that is to say their conducting cross section is designed on the basis of a multiple of the rated current. This involves considerable cost for superconducting material, particularly for the primary auxiliary windings.
The essence of the invention, in the case of a superconducting transformer, is to provide an auxiliary path, in parallel with a first superconducting primary or secondary winding, in the form of a second, normally conductive primary or secondary winding, respectively. Owing to the resistance of the normal conductor, virtually no current flows through this auxiliary or load-reducing winding during normal operation. However, if the current in the first winding exceeds the critical value in the event of a short circuit, then an electrical resistance likewise builds up there, and the alternative, normally conductive path through the second winding carries a portion of the short-circuit current.
A first exemplary embodiment of the invention is distinguished in that the first, superconducting winding is designed such that its critically current level Ic essentially coincides with the rated current IN that has to be carried on the primary side or secondary side, respectively, during normal operation.
In a second exemplary embodiment, the auxiliary winding according to the invention is located either on the primary side or secondary side, and only a first, superconducting winding is provided on the opposite secondary or primary side, respectively. This first, superconducting winding is designed such that it remains superconducting even in the event of a short circuit, that is to say it can carry several times its rated current IN.
In order to increase the impedance of the second, normally conductive auxiliary winding, this winding is provided with an inductive component, in a further exemplary embodiment. For this purpose, the second winding is wound such that a minimum distance between it and the opposite secondary or primary winding results in an additional stray inductance.
In a further exemplary embodiment, a switch is provided in series with the first, superconducting winding, which switch is opened shortly after a short circuit occurs, and interrupts the current through the superconducting winding. The latter is thus optimally protected, is heated only insignificantly, and can be reconnected to the circuit immediately once the short circuit is no longer present.